In late January, Donald Trump held one of his final rallies before the Iowa caucuses at the Dubuque airport. On the tarmac, with loudspeakers blasting the theme from the Harrison Ford movie Air Force One, a crowd of shivering supporters roared when Trump’s Boeing 757 made a flyby. Many of the estimated 400 people in attendance had been notified about the event through NationBuilder, a digital hub for campaigns that handles website design, fundraising, organizing volunteers, and social media.

As Trump has moved from outsider candidate to Republican front-runner, his campaign has been collecting e-mail addresses, cell phone numbers, and other information from supporters and feeding the data into the NationBuilder system to automate the voter-outreach process. The software lets campaign staffers target individuals with e-mails about issues in which they’ve expressed an interest and notify them of events occurring near their homes. It can also track social media so a campaign can see who’s liking or sharing a post.

NationBuilder’s technology is pretty much turnkey. It’s not as sophisticated as a custom-built platform, but candidates who subscribe to the service can immediately start tracking voters and organizing volunteers, for far less money. In the world of retail politics, the company has become a great democratizer since its founding in 2009. It’s given a political novice like Trump access to the type of sophisticated tools that Barack Obama and Mitt Romney had to build in 2012, helping Trump get his supporters to turn out for primaries and caucuses.

“This is what Obama figured out, but it took $1 billion and a whole host of engineers to do,” says Emily Schwartz, NationBuilder’s head of organizing. “Now it’s commercialized and readily available and can scale to different sizes of campaigns. You don’t have to be a fundraising machine. You don’t have to have million-dollar PACs behind you.” (NationBuilder, citing nondisclosure agreements, declined to discuss the services it provides to Trump. The campaign didn’t respond to requests for comment.)

Campaign signs for Donald Trump sit on the ground before a campaign rally at Pennichuck Middle School in Nashua, New Hampshire, on Dec. 28, 2015.

Photographer: Andrew Harrer/Bloomberg

NationBuilder is the creation of Jim Gilliam, who worked at Lycos, the search engine, before becoming an antiwar activist in the early 2000s. “He’s always been what people characterize as a hero engineer,” says Patrick Michael Kane, a former chief technology officer of MoveOn.org who now runs We Also Walk Dogs, a software company that makes the organizing software ActionKit. “He can sit down and in 10 hours bang out an application that would take another engineer 100 hours to write.”

NationBuilder’s prices start at $29 a month for e-mail blasting and social media tracking. Versions of the software that sync credit databases and consumer data—voters’ incomes, what magazines or newspapers they subscribe to, what cars they drive—with a campaign’s own voter lists run $5,000 a month and higher. Before he came along, Gilliam says, “it was at least $10,000 to get started for what we’re offering for $29 per month.” Its biggest client spends $500,000 a year.

Gilliam expected most of his customers to be state and local candidates or petition-drive organizers—such as the animal-rights advocates pushing to end carriage horses in New York’s Central Park, one of NationBuilder’s 7,000 active campaigns. “The people who are in power frankly don’t need NationBuilder,” Gilliam says. “They can afford engineers to hack things together.” But NationBuilder’s easy-to-use platform has also turned out to be attractive to candidates with plenty of money.

In 2014, Senate Majority Leader Mitch McConnell signed on for his reelection campaign. Volunteers armed with iPads scattered across Kentucky, knocking on doors with messages tailored specifically to how somebody responded to certain Facebook posts. At the end of each day, the team would review the data that streamed into NationBuilder, giving them confidence about a victory even as polls showed a tight race. “It held everything together,” says Vincent Harris, a Republican consultant who oversaw McConnell’s digital strategy.

Last year, Jeb Bush’s presidential campaign was one of the company’s largest customers among federal candidates, according to campaign spending records pulled by the nonprofit Center for Responsive Politics. Rick Santorum signed up, and so did Trump. Absent from NationBuilder’s list of customers are many Democrats. The party’s candidates rely on a company called NGP VAN, which has formal ties to the Democratic National Committee. Hillary Clinton, Bernie Sanders, and nearly every Democratic candidate for the House or Senate use the system. (Grass-roots groups supporting Sanders and others opposing Trump do use NationBuilder.)

Gilliam spent part of his childhood in San Jose, where his father was a software engineer for IBM. His parents were Christian fundamentalists and members of a local megachurch affiliated with Jerry Falwell’s Moral Majority. The family moved to North Carolina after his father was transferred, and by age 12, Gilliam was listening to Rush Limbaugh on the radio and attending church three days a week. His computer obsession started early. When his father brought home an early IBM PC with a modem, Gilliam discovered a new world. In a 2011 speech he said, “Growing up, I had two loves: Jesus and the Internet.”

He attended Falwell’s Liberty University in Lynchburg, Va., and designed its first website. “I even fixed Dr. Falwell’s computer once,” he says. But over spring break of his sophomore year, doctors discovered he had non-Hodgkin lymphoma. Two weeks into Gilliam’s chemotherapy treatment, his mother was also diagnosed with cancer. When he lived and she didn’t, Gilliam dropped out of Liberty to work for a startup in Boston. About six months later, he was diagnosed with leukemia; eventually he underwent a successful bone marrow transplant.

Gilliam went to work for Lycos in 1998. Two years later he was hired by Business.com, a search company. Gilliam rewrote the company’s main search code in 17 days and was named chief technology officer. But after the Sept. 11 attacks, his passions shifted. He was enraged by the George W. Bush administration’s invasion of Iraq and decided to make a career change after hearing that Robert Greenwald, a documentarian, needed a researcher for a film about the war.

Gilliam sent Greenwald an e-mail and was hired. He immediately demonstrated his skills at Internet organizing, creating tools to let people schedule screenings at their homes and raise money for projects. In 2005, when Greenwald released an independent film about Walmart Stores’ labor practices, the retailer hired a crisis-management firm to respond. “It was really intoxicating,” Gilliam says. “We were a ragtag group of filmmakers and did this on virtually no budget.”

Soon after, he began feeling short of breath. His earlier treatments had scarred his lungs, and he required a double-lung replacement. Surgeons at the University of California at Los Angeles decided the procedure was too risky. His friends and family organized an online campaign to change the doctors’ minds; it worked. A donor was found, and the procedure was successful. Gilliam was 29.

In late 2008, as Obama’s campaign was demonstrating what technology could do for politics, Gilliam started writing the code for what became NationBuilder. In 2010 one of his friends, Reshma Saujani, ran for Congress in New York, and Gilliam made her a database at no cost. A fundraising tool he built simplified the way people could contribute money online; one feature allowed supporters to organize events, and another made it easier for campaigns to communicate with volunteers. “I agreed to be his guinea pig,” says Saujani, who lost the race. (She now runs the nonprofit Girls Who Code.) Gilliam came away convinced he had a viable business. Others agreed: NationBuilder has raised about $35 million from the likes of Sean Parker, an early backer of Facebook, and Andreessen Horowitz. (Bloomberg LP, which owns Bloomberg Businessweek, is an investor in Andreessen Horowitz.)

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Gilliam has attempted to broaden his company’s reach outside electoral politics, with varying success. Last year it laid off about a quarter of the staff who’d been hired to strike deals with small businesses, musicians, and others in the private sector. “It didn’t work trying to push it faster than it wanted to work,” he says. Despite the setbacks, Gilliam says, the company recently became profitable. Several corporations have used NationBuilder, including Airbnb, which tapped it to mobilize customers to fight regulations. It’s also having success outside the U.S. Politicians and groups in Africa, Australia, and Europe have signed up. Britain’s Labour Party uses NationBuilder, and Canada’s leading parties have adopted the technology.

Gilliam’s friends question why he works with candidates and organizations whose political beliefs he almost certainly abhors. “I don’t want to sell to people who I think are making the world a worse place,” says Kane, the former MoveOn CTO. That line of criticism makes Gilliam angry. As he frequently points out, the right to organize is fundamental to American democracy. “Donald Trump is not the first person to use NationBuilder that I disagree with,” he says. “I probably disagree with most of our customers. That’s what democracy is about.”